1. Field of the Invention
The present invention relates to systems and methods for remote measurement of geometric features on an object, and, more particularly, to such systems and methods that are photogrammetrically based.
2. Description of Related Art
Photogrammetry is a fast, accurate three-dimensional measurement method based on photographic principles. In a single-camera or sequential mode of photogrammetry, a single high-resolution digital camera takes multiple pictures of an object from different locations. These pictures are then automatically processed to yield three-dimensional coordinates of points on an object (Ganci and Handley, 1998).
The sequential mode is not real time; so it can only measure static objects and targeted points. However, it is well suited to high-accuracy measurement of large, complicated objects, since virtually any number of pictures can be taken and processed. In addition, the photography is usually quick; so production downtime is low, and temperature effects are minimized. The use of the sequential method of photogrammetry on a typical measurement is illustrated in FIG. 1, wherein the user 90 is shown taking pictures of an object 91 from three locations 71,72,73 sequentially.
Sequential photogrammetry has other attractive attributes, including high accuracy, portability, and the ability to perform measurements in unstable or vibrating environments (Brown, 1998). All these attributes combined with the present invention are believed to make sequential photogrammetry a compelling choice for numerous applications in the automotive industry.
Three-dimensional inspection within an automotive environment has been known to be conducted within a coordinate measurement machine (CMM) room. The CMM room typically has several stationary gantry-type CMMs dedicated to a certain class of dimensional inspection. This class of inspection is most notably characterized by the need to bring the part to be measured to the CMM. Such a constraint instantly disqualifies traditional CMMs from inspecting the tooling fixtures that are located on the assembly line. These fixtures are in fact the most important items measured in the factory, as they control the dimensions and fit-up of the subassemblies throughout the manufacturing process. In-line production measurements are also impossible using CMM systems.
It would thus be desirable to have a portable system that can measure items in place for a number of applications. Although numerous portable, in-place measurement systems exist, they typically rely on touching the desired features with some type of probe or measuring adapter that is usually held in place by an operator. For many applications, access to the object is limited, making setup of the instrument difficult. Furthermore, touching the desired features with the measuring device can be difficult, tedious, and prone to lead to measurement errors.
For many automotive inspection tasks it is necessary quickly to collect and process feature data. One limiting factor in the use of photogrammetry for feature measurement in the past has been the need for point-of-interest targeting. For example, if a plane is desired to be measured, then at least three points (>3 for redundancy) need to be applied to the surface that defines that plane. Similar requirements apply to other geometric features, such as circles and lines. In some instances, point-of-interest targeting cannot be used to define the feature.
An edge is one example of a feature that cannot be directly targeted. Typically, a special target adapter is needed to define the edge 92, or the edge is determined indirectly via the intersection of two planes 93,94 (FIG. 2). The example of an edge can be extended to a corner via the addition of another plane and a line—line intersection to produce the corner. Traditionally, target adapters have been used to bypass some of the problems associated with measuring difficult features such as edges and corners. The time penalty for these adapters comes at the processing stage, when they need to be identified and reduced to yield the desired feature.
An alternative to stick-on targeting when using photogrammetry is the use of hand-held probes to touch points of interest (Ganci and Brown, 2000). Three exemplary probes 95–97 (FIGS. 3A–3C) each have a standard tip 98, similar to that used on conventional CMMs. In addition, each probe 95–97 has five permanent targets 99. In use, two or more cameras simultaneously view these targets 99 and calculate their xyz position. From the xyz position of the five targets 99, the probe tip 98 position can be calculated.
Probes have been successfully used in automotive applications for many years. However, in some applications setting up the cameras for measuring the object is difficult. Furthermore, many features are difficult to probe accurately, especially in the tight spaces and difficult conditions encountered in many measurements.
Sequential photogrammetry can often acquire data rapidly since photography is very quick. However, sometimes the need to target the desired features is difficult and time consuming. If, instead, a probing tool is used, there is typically a setup time for the instrument followed by laborious probing and analysis of each feature. The present invention is directed to reducing setup time and also to removing the requirement for probing features.